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I really fail to understand what the resistor bridge before the amplifier does. Can I just connect my inputs to the - and + ports?
This was taken from the datasheet for INA125, and they use this configuration in several of the applications, and I really can't understand why...
The bridge is there because the figure is meant to illustrate (like the caption says) a "bridge amplifier". This means its purpose is to amplify the output signal of a bridge.
If your signal source is not a bridge, then this circuit doesn't apply to you. How you should connect your inputs depends on what kind of source you have.
It's a Wheatstone bridge which is used in sensors to measure an unknown resistance. Some sensors that use Wheatstone bridges are magnetometers, strain gauges, thermistor circuits to name a few.
You can find the resistance with this equation (if there is no instrumentation amplifier the Gain term is 1) $$ V_G = Gain*(V_+ - V_-) = Gain*(\frac{R_2}{R_1+R_2}-\frac{R_x}{R_x+R_4}) $$
The nice thing about the INA125 is it has a reference voltage for the bridge. Considering the gain of the reference and gain of the instrumentation amplifier which is set by \$ R_g\$
$$ V_O = V_{ref} + Gain*(V_+ - V_-) = V_{ref} + (4+\frac{60kΩ}{R_g})*(V_+ - V_-) $$
If you don't use a Wheatstone bridge as your sensor, an instrumentation amplifier is like a multi meter, it measures the difference in voltage. So you can use the \$V+ \$ and \$V- \$ terminals to get the difference in voltage between them.
$$ V_O = V_{ref} + Gain*(V_+ - V_-) $$
Setting R_G to 60kΩ will give you a gain of 1
Keep in mind that unlike a multimeter, instrumentation amplifiers can only measure the range inside of their power rails (ground and Vcc).
